In the fields of the electrical and electronic industry, the automobile and aircraft industries, the precision instrument industry, and the like, there is a strong demand for provision of PTKs because they are aromatic polymers combining a high melting point with a high glass transition temperature and having excellent mechanical properties.
As production processes of PTKs, there have hitherto been proposed various processes in which an alkali metal sulfide and a 4,4'-dihalobenzophenone are caused to undergo a dehalogenation and sulfurization reaction. Typical patent documents on the production process of PTKs include, for example, (1) U.S. Pat. No. 4,716,212 (Japanese Patent Application Laid-Open No. 113020/1988, EP-A-0258866), (2) U.S. Pat. No. 4,795,799 (Japanese Patent Application Laid-Open No. 101332/1989, EP-A-0306884), (3) U.S. Pat. No. 5,071,944 (Japanese Patent Application Laid-Open No. 276826/1990, EP-A-0385299), (4) U.S. Pat. No. 5,097,003 (Japanese Patent Application Laid-Open Nos. 308428/1989 and 311124/1989, EP-A-0346085), and (5) U.S. Pat. No. 4,886,871 (Japanese Patent Application Laid-Open No. 54031/1989, EP-A-0293115).
In U.S. Pat. No. 4,716,212, there is disclosed a process for obtaining a high-molecular weight PTK, in which an alkali metal sulfide is used in excess of a 4,4'-dihalobenzophenone. The polymer obtained by this process is inadequate in melt stability and is in the form of fine powder having an average particle diameter of 20 .mu.m or smaller.
U.S. Pat. No. 4,795,799 discloses a process for obtaining a high-molecular weight PTK, in which an alkali metal carbonate is caused to coexist in a polymerization system. According to this process, a granular polymer having an average particle diameter of 0.1 mm or greater can be obtained. However, the polymer formed is extremely insufficient in melt stability.
U.S. Pat. No. 5,071,944 discloses a production process of a PTK, in which in the first polymerization step, dehydration is conducted until the water content in a polymerization reaction mixture is reduced to about 0.1-10 wt. % based on the weight of a polar organic solvent and in the second polymerization step, polymerization is then carried out in a state of the low water content. According to this process, a polymer having a high molecular weight can be obtained. However, the polymer formed is in the form of fine powder because of the polymerization condition of such a low water content.
In U.S. Pat. No. 5,097,003, there is disclosed a production process of a PTK, in which a reaction is conducted in a temperature range of as low as 80.degree.-170.degree. C. in an initial stage of polymerization without removing free water and hydration water in an alkali metal sulfide in a polar organic solvent prior to the polymerization by the time the residual rate of an unreacted dihalogenated compound becomes 2% or lower, and thereafter in a temperature being raised up to the range of 200.degree.-345.degree. C. According to this process, preliminary polymerization is conducted at a low temperature as low as 170.degree. C. or lower. Therefore, unreacted monomers, low-molecular weight oligomers and the like remain, so that deleterious side reactions tend to occur in final polymerization, resulting in difficulty in obtaining a polymer good in melt stability. In addition, the long-time reaction at the low temperature in the preliminary polymerization is disadvantageous from the viewpoint of productivity.
U.S. Pat. No. 4,886,871 discloses a process for obtaining a PTK good in melt stability, in which polymerization conditions such as a water content, a charge ratio of monomers and a polymerization temperature are suitably to be selected. However, this process requires to control the polymerization temperature within a range of 60.degree.-300.degree. C., preferably 150.degree.-290.degree. C., more preferably 220.degree.-280.degree. C. It is therefore difficult to obtain a granular polymer with practical yield. For getting a granular PTK, a polymerization reaction system is to be heated to a temperature at least higher than 300.degree. C. in the presence of a relatively great amount of water in the final stage of the polymerization.
The melt stability of a PTK can be evaluated by its maintenance of crystallinity after holding the polymer for a certain period of time in a molten state, for example, the retention of melt crystallization enthalpy (.DELTA.Hmc). If the PTK is low in retention of melt crystallization enthalpy and hence poor in melt stability, conventional melt processing techniques such as extrusion, injection molding and melt spinning can not be applied to such a polymer.
If a polymer formed is in the form of fine powder on the other hand, difficulty of the handling of the polymer is significant upon filtrating, washing, drying, transporting, or charging of the polymer during the polymer collection process after polymerization, so that expensive equipments, complicated operations, long treatment time and the like are required. Further in forming or molding and processing, the finely powdered polymer is difficult to handle and tends to cause many troubles such as making weighing difficult or forming block in a hopper, and hence requires an extruder equipped with a compactor or the like. Therefore, it is extremely important from the viewpoint of productivity and economy to obtain a polymer in the form of granules from a polymerization reaction system.
As a method of granulating a PTK after completion of the polymerization reaction, there has been proposed in EP-A-0311040 (Japanese Patent Application Laid-Open Nos. 129027/1989 and 315420/1989) a treatment process in which the PTK is heated in a polar organic solvent to liquefy it, the resulting liquid mixture is added with a separating agent such as water, and the temperature is lowered to solidify the polymer. However, this treatment process involves a problem that the polymer is subjected to significant deterioration, when the PTK is dissolved molecularly and exposed to such a high temperature as over 300.degree. C. before added the separating agent. Therefore, such a treatment process lowers the melt stability of the PTK and is hence hard to provide a PTK sufficiently high in retention of melt crystallization enthalpy.
As described above, it has been very difficult to provide a PTK having high melt stability as granules so far.